Local equilibrium of mafic enclaves and granitoids of the ... · American Mineralogist, Volume 76, pages 574-588, 1991 Local equilibrium of mafic enclaves and granitoids of the Turtle

American Mineralogist, Volume 76, pages 574-588, 1991

Local equilibrium of mafic enclaves and granitoids of the Turtle pluton,southeast California: Mineral, chemical, and isotopic evidence

CH.q.RLorrr M. Ar,r,rN*Department of Geological Sciences, Viryinia Polytechnic Institute and State University, Blacksburg, Virginia 24061 U.S.A.

Ansrnq.cr

Major element and trace element compositions of whole rocks, mineral compositions,and Rb-Sr isotopic compositions of enclave and host granitoid pairs from the Early Cre-taceous, calc-alkaline Turtle pluton of southeastern California suggest that the local en-vironment profoundly affects some enclave types. In the Turtle pluton, where the sourceof fine-grained, mafic enclaves can be deduced to be magmatic by the presence of partiallydisaggregated basaltic dikes, mineral chemistry suggests partial or complete local equilib-rium among mineral species in the enclave and its host granitoid. Because of local Rb-Srisotopic equilibration between fine-grained enclaves and host granitoid, one cannot use Srisotopes to distinguish an enclave source independent of its host rocks from an enclavesource related to the enclosing pluton. However, preliminary Nd isotopic data suggest anindependent, mantle source for enclaves.

INrnonucrroN

Fine- to very coarse-grained mafic rocks commonly oc-cur in calc-alkaline plutons as enclaves (inclusions) anddikes (Didier, 1973,1987; Reid et al., 1983; Vemon, 1983;Cantagrel etal., 1984; Frost and Mahood, 1987; Kumar,1988). Although mafic enclaves vary in composition,mineral content, and texture, fine-grained enclaves inmetaluminous to weakly peraluminous rocks share nu-merous features (see references above). There are severalmodels to explain the source of enclaves and the rela-tionship of enclaves to the surrounding pluton. Modelsof chemical evolution from diorite to granite invoke frac-tional crystallization or removal of restitic or source ma-terial (Bateman et al., 1963; White and Chappell, 1977;Bateman and Chappell, 1979; Chen et al., 1989). Othermodels propose mixing and mingling of mantle-derivedmafic magma and crust-derived granitic magma to gen-erate the range of granitoid and enclave compositions (forexample, Barbarin, 1988; Stewart et al., 1988). Field ob-servation of partially disaggregated, synplutonic, basalticdikes supports the mixing-mingling model (for example,Reid et al., 1983; Barnes et al., 1986; Frost and Mahood,1987; Kumar, 1988). Lastly, some mafic enclaves arethought to be xonoliths ofolder plutonic rocks (Jurinskiand Sinha, 1989).

The ability to decipher whether mafic enclaves in agiven pluton are representative of (l) early crystallizingliquids, or (2) magmas that have contributed to the evo-lution of a pluton on a gross scale through mixing, or (3)unrelated, accidental magmas and xenoliths that have notaffected granitoid compositions (except locally) is contin-

t Present address: Department of Geological Sciences, Uni-versity of Kentucky, Lexington, Kentucky 40506-0059, U.S.A.

0003-004x/9 I /0304-05 74$02.00

gent on the degree of physical and chemical interactionbetween enclaves and host granitoid magmas, i.e., thelack of local equilibration of enclaves with surroundinggranitoid.The preservation of synplutonic mafic dikes inplutons and their textural similarity to fine-grained en-claves suggest an igneous origin for these enclaves; how-ever, enclaves commonly display a very similar miner-alogy and crystallization sequence to those oftheir granitichost (Vernon, 1983). This suggests equilibration of themafic enclaves and host granitoid. Equilibration is thoughtto be a complex process. Exchange can be accomplishedby magma mixing, or by chemical exchange through dif-fusion or reaction or both. Exchange can be between mag-ma and crystals, between fluid and crystals, or betweencrystals (Eberz and Nicholls, 1990). In this study, thedegree of chemical interaction among enclave types andtheir host pluton, and the amount of source informationretained by enclaves, is examined using data from themesozonal, 130-m.y. Turtle pluton of southeastern Cal-ifornia (Fig. I and Table l; Allen, 1989). This study uti-Iizes field relations, whole rock geochemistry, Rb-Sr andNd-Sm isotopes, and mineral compositions.

Frnr-o oBSERvATToNS

The Turtle pluton is a revorsely zoned intrusion, mean-ing it has a granitic to granodioritic rim and more mafic,granodioritic core (Fig. 1; Allen, 1989). It can be dividedinto four facies. The rim sequence is an arcuate unit thatgrades inward from biotite granite to hornblende-biotitegranodiorite. The schlieren zone is a zone ofhigh straintens of meters wide along the contact of the rim sequenceand core facies. The core facies is a relatively homoge-neous unit ofbiotite-hornblende granodiorite and qtarlzmonzodiorite. The fourth facies is exposed in the FourDeuce Hills and forms the eastern lobe of the Turtle plu-

574

ALLEN: MAFIC ENCLAVES IN GRANITOIDS 575

27 *'ttu

t r rn so' covcrco

TURTLE PLUTONGfonitoids

O Finc-groincd cnclovcs

A Coorsc-groincd cnclovcr (Kd)

lMo l ic d ikcs (d )

p€ Precombrion country rocks

r, t q\r175

Fig. 1. Simplified geologic and sample location map.

ton. It consists of granite and granodiorite with mineralcontent and composition like the other facies, but thisfacies has an unknown intrusive relationship with the restof the pluton. The Turtle pluton is intruded by two sig-nificantly younger intrusions-the Fortification grano-diorite and Target granite, and the pluton intrudes thePatton $anite of unknown age (Allen, 1989).

Three types of enclaves occur within the Turtle pluton:xenoliths, fine-grained microgranitoid enclaves, andcoarse-grained dioritic to gabbroic enclaves. Xenoliths areangular blocks ofquartzofeldspathic gneiss, like the sur-rounding Precambrian country rock. These xenoliths arefound only within tens of meters of the contact of thepluton with the country rock and within the schlieren

TABLE 1. Sample descriptions

Sample No. Rock type

Bt graniteBt graniteBt granite and fine-grained enclave

HbLBt granodioritefine-grained enclave in CA84-4

Hbl-Bt granodioritefins.grained enclave in BW8+23

Bt-Hbl granodioritefine-grained enclave in BW84-25coarss.grained enclave in BW84-25fine-grained enclave in Bt-Hbl granodioritefine-grained enclave in Bt-Hbl Qtz monzodioritecoarse-grained enclave in Bt-Hbl granodioritecoarse-grained enclave in Bt-Hbl granodiorite

,Vote.'Adiacent samples marked by brackets. Bt : biotite. Hbl : horn-blende.

Fig. 2. Field photographs of(a) typical fine-grained enclaveswarm in granodiorite (outcrop 3 m across), and (b) synplutonicmafic dike (sample CA85-4C) intruded by aplites and offset onlow angle normal faults. Hammer handle is 48 cm long.

zone. This type ofenclave has sharp contacts and showsno evidence of assimilation; it is not considered in furtherdiscussion. Fine-grained microgranitoid enclaves arefound in all units, either singly or in swarms (Fig. 2a),and this enclave type composes 4-50lo by area of the plu-

ton based on measurements of enclave numbers and sizes(Waugh, 1985; Allen, 1989). They are characterized byaverage grain size less than 3 mm, by greatest observeddimension less than 2 rn, and by discoidal shape althoughdigitate shapes are present (Fig. 2a). Contacts are gener-

ally sharp. These enclaves are basaltic to andesitic incomposition and are common in all outcrops. Coarse-grained enclaves principally occur in a few regions withinthe pluton as ovoid to angular blocks (see Fig. l) but arepresent in all facies. This enclave type probably composesa similar proportion of the pluton as the fine-grained va-riety, but its distribution is much different, as noted above.Coarse-grained enclaves arc chancreized by average grain

size greater than 5 mm and by smallest dimension greater

cA85-5BW84-20BW84-20AcA85-4CA84.45BW84-23BW84-29BW84-25BW84-254sw84-27cA84-1 13cA84-173cA84-1 1 4cA85-1 1 I

576 ALLEN: MAFIC ENCLAVES IN GRANITOIDS

c

h

* - . ' t i

t i

r l

-lcm

Fig' 3. Photograph of thin sections showing the textural variety of mafrc rocks: (a) hornblende-free, frne-grained enclave ingranite near wall rock contact; (b) and (c) fine-grained enclaves in granodiorite; (d) synplutonic mafic dike, CA84-65A; (e){h) coarse-grained enclaves, BW84-27, CA84-114, CA85-118, and example of cumulus texture, respectively.

than 2 m. Rock type and texture vary from coarse-grainedhornblende + clinopyroxene diorites and gabbros tohornblendites with crystal size in the centimeter range.

Aside from aplitic and pegmatitic dikes that intrude allfacies of the pluton, one other kind of synplutonic dikewas recognized by mutually cross-cutting relationshipsvdth host granitoids. These dikes are basaltic, and theyrange in thickness from a few centimeters to 2 m (Fig.2b). Some have finer-grained margins. These dikes onlyoccur in the periphery of the pluton (Fig. l) and, in mostcases, they are texturally and mineralogically very similarto some fine-grained enclaves (Figs. 3b-3d). A geneticrelationship ofdikes to fine-grained enclaves is probablegiven the textural similarity and field evidence for dikedisaggregation (Fig. 4). Thus a model of enclave forma-tion from disaggregation of basaltic dikes is favored forthe fine-grained enclaves ofthe Turtle pluton (though a

specific source like sampled dikes for all enclaves is notintended). Field evidence, such as crosscutting relation-ships and intermediate textural types, that could aid inunderstanding the relationship of coarse-grained enclavesto either the fine-grained enclaves or the granitoids oftheTurtle pluton was not observed.

FrNn-cnLrNED ENcLAvEs

The mineralogy of the fine-grained enclaves reflects thatof their host granitoids. These enclaves contain the min-eral assemblage plagioclase (Pl) + quartz (Qtz) + potas-sium feldspar (Kfs) + biotite (Bt) + aparire (Ap) + horn-blende (Hbl) + magrretite (Mag) t ilmenite (Ilm) +titanite (Ttn) + zircon (Zrn). Enclaves in ilmenite-biotitegranite lack hornblende, magnetite, and titanite, whereasthose in magnetite-titanite-biotite-hornblende granodio-rite contain the same assemblage as the host granodiorite

42F |.,

/,<4 o- / Lt\t

t ,o""r"d

ALLEN: MAFIC ENCLAVES IN GRANITOIDS 577

Fig. 5. Photomicrographs showing typical textures of fine-grained enclaves, particularly (a) hypidiomorphic-granular tex-ture with acicular hornblende (photo 5 mm across), and (b) com-mon acicular apatite, which is segmented in many cases (photo1.8 mm across).

crystallized early as a result of thermal (Wyllie et aI., 1962)or compositional undercooling (Lofgren, 1974). Second-ary minerals [chlorite (ChD + epidote (Ep) + sericite tcalcite (Cal)l generally compose less than 5olo of this rocktype.

Fig. 6. Crystallization sequences for mafic rocks and granit-oids. Minerals given in parentheses may be present or absent ina given sample.

Fig. 4. Line drawing of foreshonened photograph of disag-gegated mafic dike and onclaves (both stippled) in granodiorite.Bar in center : 15 cm.

(Table 2). The color index of these enclaves is generally35 to 55 (Table 2, Figs. 3a-3d) and textures are hypidi-omorphic-granular (Fig. 5a). Because this texture is com-mon in granitoid rocks, Vernon (1983) applied the termmicrogranitoid to mafic enclaves. Enclaves commonlycontain phenocrysts (xenocrysts?) of plagioclase, q\artz,or hornblende (Fig. 5a). Crystallization sequences basedon petrographic observation of grains other than pheno-crysts are presented in Figure 6. Plagioclase occurs assubhedral to euhedral crystals which have oscillatory zo-nation and an overall nornal chemical trend. Some pla-gioclase shares mutual I 20" junctions, suggesting texturalequilibrium. Quartz is subhedral to anhedral and has un-dulose extinction and subgrain development. Potassiumfeldspar is interstitial. Biotite is brown pleochroic andoccurs as subhedral flakes dispersed throughout the rockand as ragged flakes in hornblende. Hornblende is acic-ular and green pleochroic and commonly contains opti-cally continuous inclusions ofbiotite that suggest a bio-tite-consuming reaction. Opaque minerals (primarilymagnetite) have a blocky morphology, and titanite is sub-hedral and commonly contains opaque minerals. Euhed-ral zircon is present in some specimens. Apatite is anubiquitous accessory mineral and is acicular (length: widthup to 100:1), commonly segmented, and typically con-tains fluid inclusions (Fig. 5b). Some grains with Y-ter-minations were observed. Acicular apatite is recognizedas a common feature of microgranitoid enclaves (Didier,1973; Vernon, 1983; Reid et al., 1983), and its morphol-ogy and inclusion in all other major minerals suggest it

FINE-GRAINED MAFIC COARSE-GRAINED HOST

corly----+ lotc "orly

----) lote eorly---| lotc .orly-----' lotc

578

TABLE 2, Modal analyses

ALLEN: MAFIC ENCLAVES IN GRANITOIDS

Sample Type Kfs cpx EpApopq

BW84-20 granite8445 tg enclCA85-4 gdiorite85-4C m dikeBW84-29 fg enclBW84-23 gdioriteBW84-25A fg enclBW84-25 gdiorite84-27 cg encl84-113 fg encl84-173 fg encl84-118 cg encl84-114 cg encl84-65A m dike

43.5 23.139.8 0.446.4 18.221.4 039.8 0.654.5 9.946.2 0.557.6 7.129.6 0.6s5.8 0.344.6 0.731.8 0.026.1 0.930.1 0

29j 3.s 03.8 11.0 41.1

24.3 7.1 2.82.2 4.2 44.6

10.7 11.1 26.121.1 9.8 3.71 .1 11.3 34.8

17.7 8.8 6.96.2 0.2 51.20.4 19.0 20.93.8 11.0 29.0

10.4 1 .2 41.48.9 8.8 49.54.8 0.3 37.5

0 0.40.1 0.20.4 0.21 .8 0 .21 .2 0 .30.3 0.31 . 1 00.5 1.01 .6 0 .20 .9 1 .10.9 0.61 .2 00 00.7 0.3

0 00 00.1 00.8 00 00 0.20 00 0.10 00.0 00.2 00.0 00.2 00.1 0

0.3 02.8 0.40.3 0

18.4 0.210.0 00.1 0.13.1 0.30.1 0.17.O 00.9 0.66.5 2.8

12.6 0.45.1 0.5

18.9 3.3

0.1 00.4 00 .1 05.6 0.60.1 00 01 . 6 00.1 00.4 00 0.10 01 . 0 00 04.1 0

000000003.000000

Note: Pl: plagioclase, Kfs : potassium feldspar, Otz : quartz, Bt: biotite, Hbl : hornblende, Cpx = clinopyroxene, Ttn : titanite, Opq : opaqueminerals, Ap : apatite, Aln : allanite, Seric : sericite, Ep : epidote, Chl : chlorite, Cal : calcite.

Co,Lnsn-cn c.rNED ENCLAvES

Coarse-grained enclaves range in composition fromhornblende diorite and gabbro to hornblendite, and theycontain the assemblage Pl + Qtz + Kfs + Hbl + Bt +Mag + Ttn + Ap t Clinopyroxene (Cpx). This assem-blage is independent of the mineral content of the sur-rounding granitoid (Table 2). The color index varies from

Tlele 3. Whole rock geochemistry

approximately 43 to 55. Hornblendite was not analyzedchemically and is not considered in further discussion.Coarse-grained enclaves contain tabular, normally zonedplagioclase. Potassium feldspar is interstitial to all otherminerals. Quartz, a conspicuous modal mineral in theseenclaves, displays undulous extinction and subgrain de-velopment and appears to be in textural equilibrium withsurrounding minerals. Hornblende is prismatic and green

SampleRock type

Error('t"l

BW&L2Ogranite

cA8+45fg encl

c4854gdiorite

cA85-4Cm dike

BW84-29fg encl

BW84-23gdiorite

sio,Tio,AlrosFeOMnOMgoCaONaroKrOP,O.LOISUMBa (ppm)Rb (ppm)Sr (ppm)

ANoOrAbAncDiHyolt lAp

111133131

< 1<5

31 . 51 .5

73.32o.20

14.551.490.080.461.873.923.470.090.30

99.751 100102276

20.7631.6320.5133.178.691 . 1 60.003.700.000.380.21

1.080.710210.70820.11240.51 2259

-7.390.5121 6

-6.00

50.830.98

16.2010.330.486.857.332.102.000.291.54

98.93139108284

61.900.00

11.8217.7728.870.004.64

28.283.481.860.67

1 . 1 00.708940.7069

C,l.P.W. nonn3

68.180.36

15.412.620.091.253.353.733.720.150.63

99.49985112357

0.900.708160.7065

52.091.52

15.479.170.157.059.502.U1.550.360.96

100.16

54473

57.810.009.16

19.8027.130.00

14.5124.480.402.890.83

0.310.705480.7049

53.351.2' l

18.317.510.325.007.522.741.900.300.76

98.9219091

346

58.032.49

11.2323.1932.050.002.73

23.490.002.300.70

0.760.70trt30.70690.13210.512478

-3.120.51237

- 2.05

66.900.45

16.313.430.101.774.123.162.820.190.84

100.0968890

439

41.7924.1816.6726.7419.201.020.00

10.150.000.850.44

0.600.70823o.70710.12020.512375

-5 .130.51227

-3.86

31.0021.9221.9831.5614.320.001 . 1 16.940.000.680.35

lsotopic datasiRb/86Sr87Sr/665r

SRI147sm/l4Nd143Nd/14Nd

e N d1€Nd/'4Nd(t)

e Nd(t)

10.015

0.200.002

Note.'Maior elements in wtyo.

pleochroic. One gabbro was observed to contain clino-pyroxene in reaction relationship with rimming horn-blende (BW8a-27); no clinopyroxene was observed inTurtle pluton granitoids. Brown pleochroic biotite com-monly occurs adjacent to or within hornblende. Acces-sory phases are titanite, subhedral to anhedral opaqueminerals, and acicular and prismatic apatite. Secondaryminerals (Chl + Ep + sericite) generally compose muchless than l0o/o of most of the samples. The fundamentaldifferences between fine- and coarse-grained enclaves arethe sizes ofindividual enclaves, grain size, and the pres-ence of clinopyroxene in coarse-grained enclaves.

Mnrrc nrrnsMafic dikes have color indices greater than 40 and con-

tain Pl * Qtz + Kfs + Hbl + Bt + Mag + Ttn + Apregardless of the mineral content of surrounding graniteor granodiorite (Table 2). In many cases, these rocks arelithologically indistinguishable from fine-grained micro-granitoid enclaves that lack phenocrysts (see Figs. 3b and3d) except for the lower abundance of biotite and thegreater abundance ofsecondary phases, Ep + Chl + Cal+ sericite (approximately 20-25o/o, principally sericite af-ter plagioclase), in the dikes. The petrographic similarity

TABLE 3-Continued

ALLEN: MAFIC ENCLAVES IN GRANITOIDS 5't9

of dikes to fine-grained enclaves includes rare examplesof biotite mantled by hornblende.

Wrroln RocK GEocHEMrsrRY

Whole rock, major element, Rb, Sr, Ba, Rb-Sr isotopicand Nd-Sm isotopic analyses of fine-grained enclaves,coarse-grained enclaves, host granite, and granodiorite aregiven in Table 3. The average dimension of fine-grainedenclaves sampled for whole rock chemistry averaged about40 x 40 x 20 cm. The outer l-2 cm region of the enclavewas not analyzed. Coarse-grained enclaves (>4 m in min-imum dimension) and dikes (about l-2 m in thickness)were sampled from core regions. Host granite and grano-diorite were sampled 10-50 m from analyzed enclaves,and were free from recognizable enclaves. Major and traceelement analyses were performed by XRF using wave-length dispersive techniques modified from Norrish andChappell (1977). Rb-Sr isotopic ratios were determinedon a modified 35 cm radius, 90o degree sector, Avco spec-trometer at Virginia Polytechnic Institute and State Uni-versity (VPI and SU). Samples were loaded on single Refilaments with H3PO.. All isotopic data are normalizedto the Eimer and Amend SrCO, standard value of 0'70800from an average value of0.70813(n:23, standard enor

BW84-25Afg encl

BW84-25gdiorite

BW84-27cg encl

cA85-1 1 Icg encl

cA84-1 14cg encl

cA84-654m dike

cA84-113 CA84-173fg encl fg encl

52.021 . 1 0

18.608.520.256.718.902.021.860.451 . 1 3

101 .5629658

503

67.920.00

10.9917.0936 190.004.13

28.680.212.091.04

0.330.707720 70710.12830.512512

-2 460.51240

-1 .33

64.960.53

16.564.410.101.955.222.852.190.230.51

99.5178453

547

50.2923.1812.9424.1224.390.560.00

12.270.001.010.53

0.280.706980.70650.1 205o.5124403.860.51234

-2.60

52.591 . 1 7

15.358.690 .197.62

10.641 .870.87o.201 . 1 4

100.3316321

376

66.152.685 .14

15.8230.920.00

16.8525.080.002.220.46

0 .160.706300.70600.14840.51 2605

-0.640.512480 .16

51.270.98

18.709.200.204.848.412.611 .61o.440.85

99 .1118666

s24

9.5122.O934.56

0.003.58

24.880.771 .86

50.321 . 1 6

1 8 1 9'to.210.284.519.443.331.210.410.48

99.54

28507

7.1528.183 1 . 1 10.00

10.955.24

13.272.20

54.791.26

15.037.930.166.61

10.011.902.010.261 . 1 7

1 0 1 . 1 3

46412

62.284.28

1 1 . 8 816.0826.550.00

17.5720.610.002.390.60

0.320.705550.7050

54.750.58

12.209.010.309.969.171.321.850.261.38

100.78

64234

66.233.61

10.9311.1721.900.00

17.8732.230.001 . 1 00.60

0.790.707020.7056

51.401.27

17.269.930.265.529.142.441.710.401.30

100.6349555

613

60.100.00

1 0 . 1 120.6531.090.009.69

21.393.072.410.93

0.260.706000.70550.1 3290.51 2489

-2.910.51238

-1 .85

C.l.P.W. norms61.01 52.48

0.00 0.00

1.O2 0.95lsotopic data

0.36 0.160.70737 0.706720.7067 0.7064

0.14280.51251 8

-2.340.51240

- 1.45


of mean : 0.00001). Total blanks for Rb and Sr were lessthan 0.2 ng. Nd-Sm isotopic ratios and elemental con-centrations were detormined on a Finnigan-Matt 262 atthe U.S. Geological Survey, Menlo Park, California andno bias corrections are necessary. Isotopic analyses weredetermined on samples loaded with HNO3 and HrPOoonto Re in a double Re filament configuration and ob-tained in a dynamic switching mode (143, 144, 145-144,145, 146). Multrple analyses of BCR-l yield'ar\d/t+efr[d: 0.512633 + 10 (950/o confidence level). '43Nd//'44Nd wasnormalized to r46Nd//'44Nd. Present day chondritic valuesused are r43Nd//r44Nd : 0.512635 and 147sm/r44Nd :0.1967. A mixed '4esm/rsoNd spike was used and massratios were obtained in a static mode. Total blanks forNd and Sm are less than 0.1 ng. Analytical errors arelisted in Table 3.

The Turtle pluton grades in composition across the rimsequence and schlieren zone, and into the core facies. Theranges of composition for this reversely zoned granitic(rim) to granodioritic (core) pluton are 73.32-6L 68 wto/oSiOr, 102-56 ppm Rb, 275-600 ppm Sr, 875r,/865r at 130Ma (hitherto SRI) of 0.7083 to 0.7065, and e*u(t) (whereI : 130 Ma) of -6.00 to -2.60 (Table 3, Figs. 7 and 8).There is a strong linear correlation among time-correctedisotopic ratios (Fig. 8) and of isotopic ratios and mostmajor element abundances of these granitoids. These datahave been modeled as magma mixing and concomitantfractional crystallization (Allen, 1989, and in prepara-tion). The facies exposed in the Four Deuce Hills doesnot fall on trend with other facies on Rb-Sr isotopic plot,but similarity of SRI of the rocks of the Four Deuce Hillsand the core facies suggests the former is likely to be afractionate ofthe latter (Fig. 8a).

The chemical contribution of mafic enclaves and dikesto granitoids of the Turtle pluton will be evaluated withreference to linear arrays (bulk mixing lines) on isotopicplots.

Fine-grained enclaves are olivine normative, medium-to high-K,O basalt (SiO, : 50.32-52.02 wo/o\ with theexception of one high K,O andesite (53.35 w0/o; termi-nology from Gill, l98l). Coarse-grained enclaves arequartz normative, medium- to high-KrO basalt to andes-ite that contain more SiO, (52.59-54.79 wto/o) than thefine-grained variety. Mafic dikes are high-KrO, olivinenormative basalt (SiO, : 51.40-52.09 wto/o). Based onmajor and trace element data and SRI, the three maficrock types fall into three chemical groupings. Fine-grainedenclaves and mafic dikes overlap in major and trace el-ement composition, but dikes have significantly lower SRIratios and higher TiO, than enclaves (Fig. 8a and Table3).

In view of the field relationships that suggest fine-grained enclaves result from dike disaggregation (Fig. 4),the fact that the enclaves have the more radiogenic SRIpoints to interaction with host granitoids. This interac-tion is supported by the correlations of SRI and by theKrO and Rb contents ofenclaves and nearby host gran-itoids (Table 3, Fig. 8a). In particular, the similarity of

Rb contents of enclave and host is striking (< 100/o differ-ence; Fig. 7). On the other hand, nonoverlapping e*u(l)(Fig. 8b) suggest discrete origins of enclaves and granit-oids of the Turtle pluton. The complications caused bypluton-enclave interaction in assessing whether enclavesare restite, magmas that contributed to evolution of thepluton, or accidental magmas is discussed below.

Coarse-grained enclaves contain more SiOr, CaO, andMgO, and less AlrOr, than the other mafic rock types;they have SRI less than that offine-grained enclaves andgranitoids, but SRI similar to those of the dikes. Thechemical dissimilarity of the two enclave types suggestsdistinct histories. These mafic rocks have very differentRb concentrations than those of host granitoids, unlikethose of the fine-grained enclaves. The low SRI (<0.7060)and high e"u(/) (+0.16) of the coarse-grained type andlack of overlap with Turtle pluton granitoid isotopic val-ues indicate (1) less crustal interaction in formation ofthis type than in the formation of fine-grained enclavesand (2) perhaps little or no interaction with the surround-ing pluton. No model of assimilation and fractional crys-tallization relating fine-grained enclaves, granitoids, andcoarse-grained enclaves can be derived from the availablegeochemical data.

The following generalizations can be made about maficrocks and adjacent host rocks (field groupings given inTable I and Figs. 7 and 8). Of the fine-gained enclaves,the ones in the more evolved host granitoids contain moreKrO and Rb, less Sr, and variable concentrations of othermajor elements and Ba (Table 3). The SRI and Rb con-tents of most of these fine-grained enclaves are similar tothose of their hosts (Table 3), and in one case the SRI ofthe enclave is significantly gireater (BW84-25A). Data forone coarse-grained enclave and its host granodiorite, andone mafic dike and its host, indicate that these mafic rocktypes have a much lower SRI than their hosts. The e*o (l)for both mafic enclave types and a mafic dike are allg.reater than the range observed for granitoids.

MrNnn-qr, cHEMrsrRy

Biotite, hornblende, plagisglssr, and apatite from maf-ic rocks and host granitoids were analyzed with an au-

1506 Granitoidsa Fine-gr enclA Coarse-gr encl.I Mafic dikes

Sr

Fig. 7. Plot of Rb vs. Sr in ppm. Lines connect paired maficrock-host ganitoid samples.

1-ocr


TABLE 4. Kakanui hornblende standard analyses

581

StandardMean deviation

Standarddeviation

o/o2STDl W€tmean chemistry

SiAlbt

TiFeMnMgCaNaKFOHoFe no.

sio,Alro3Tio,FeOMnOMgoCaONa.OKrOFHrO'- o : FSUM

40.8414.204.83

10.510.10

13.051 0 . 1 12.502.060.191.960.13

100.23

0.660.300.150.140.030.240.180.200.050.050.02

0.85

5.9722.4470.5321.2860.0122.8461.5830.7090.3850.0851 .915

24.0000.311

0.0500.0s00.0190.0200.0030.0570.0250.0s60.011o.022o.022

0.006

1.684.096.963 .17

52.173.993 .16

15.885.49

51.56

5.9002.5700.5201.3400.0102.7801.6100.8000.380

Note: n: 40.. Calculated, assuming F + OH : 2, molecular.

tomated, nine-spectrometer, ARALSEMQ electron mi-croprobe at VPI and SU using silicates and oxides asstandards in an analytical scheme called QALL (Solbergand Speer, 1982). Analytical errors are based on multipleanalyses ofa Kakanui hornblende standard, one not usedin calibration (Table 4). Representative and average anal-yses of minerals appear in Tables 5 through 8.

Biotite

Biotite is the only ferromagnesian silicate that occursin all facies of the Turtle pluton and in all mafic rocks.As can be seen from representative plots of biotite com-positions (Table 5 and Fig. 9), biotite from granite andfine-grained enclaves that contain ilmenite (outer portionof the pluton) are distinct in composition from biotite inmagnetite-bearing samples, but in all cases, compositionsof biotite from fine-grained enclaves overlap the com-positions of biotite from their host granitoid. A maficdike (CA84-65A) contains biotite with a compositionsimilar to those of fine-grained enclaves but with Fe/(Fe+ Mg) at the high end and F at the low end of the rangesobserved for the enclaves. In contrast, data from onecoarse-grained enclave are different with Fe/(Fe + Mg)of 0.377-0.381 (not shown).

These data srrggest biotite in fine-grained enclaves at-tained equilibrium with the surrounding granitoid. Be-cause enclaves contain a significant proportion ofbiotite(10-20 modal TQ, equilibration of biotite can profoundlyaffect the whole rock composition, especially those ele-ments strongly partitioned into biotite, namely K, Rb,Cs, Ba, Sc, and LREE.

Amphibole

Amphibole from mafic enclaves and dikes and hosts iscalcic magnesio-hornblende, with the exception of someamphibole samples from a mafic dike (CA84-65A) thatare edenite (Na + K > 0.50; classification scheme ofHawthorne, I 98 I ; Table 6; Fig. l0). No consistent opticalor chemical zonation of amphibole is apparent except forfive analyses of amphibole immediately adjacent to bio-tite inclusions in an enclave (BW84-29). These amphi-

0.7090

0.7080

t=

!? 0.7070

NU)@

0.7060

0.70s0

a.

,6"Y

ho/ tust

oz$ 0.s123

E

gz o.s'tn

0.00 0.20 0.40 0.60 0.80 1.00 1.20

0.5125

0.5124

0,s121

0.5128.b0 . 1 1 0 j 2 0 . 1 3 0 . 1 4 0 . 1 5

totsm/tooNd

Fig. 8. Time-corrected isotopic plots of Rb-Sr and Nd-Smsystems. Symbols as in Figure 7. Mixing lines are linear regres-sions ofavailable data for Turtle pluton granitoids (fields) exceptfor anomalous granitoids from the Four Deuce Hills (CA85-4).Most enclave data do not lie along mixing lines. See text fordiscussion.


TABLE 5. Biotite compositions

BW84-20A BW84-20A BW84_23 BW84_25Bt granite f.g. enclave granodiorite granodiorite

Avgn : 4 O

std Avgn : 7 7

std Avgn : 2 2

std Avgn : 2 5

sio,Al2o3Tio,FeOMnOMgoCaONaroKrOFH"o- o : F

Total

36.2216.052.66

19.201.249.420.040.069.630.893.500.46

98.45

5.4712.5290.4980.3472.3060.1622.1790.01' l0.0621.7930.4180.515

0.820.680.490.600.100.480.020.080.360.080.10

1.00

0.1920.1920.2650.1030.167o.o220.190o.0440.0470.1230.1010.015

35.9416.272.69

18.291.26

1 .0597.55

37.3315.533.22

17.390.63

1 1.94o.o20.089.370.203.870.11

99.50

5.6272.3730.3850.3642.1910.0802.6830.003o.0241.7990.0970.450

0.450.500.460.790.060.430.050.02o.700.030.06

1.97

0.0400.0400.0820.0490.0830.0080.1310.0080.0070.1 180.0150.017

37.0515.312.91

18.770.53

11 .610.060.139.200.223.820.07

99.54

5.6162.3840.3490.3812.3800.0672.6240.0100.0381.7780.1080.476

0.460.310.470.420.050.430.060.050.340.160.09

0.87

0.0490.0490.0&40.0520.0560.0070.1050.0100.0140.0590.0750.013

Compositions in wt060.700.560.390.530.10

9.84 0.370.02 0.010.2't 0.099.34 0.350.92 0.093.82 0.08

SirltAlr6jAl

TiFeMnMgGaNaKrFe(Fe + Mg)

1 .24Cations per 24 O atoms

5.510 0.0672,490 0.0670.449 0.0870.3102.3460.1642.2490.0030.0621.827

o.o440.0610.0130.0780.0020.0270.061

0.443 0.0410.511 0.010

enclave

P204.7

)I^ - _

65A ln\

t ,' 1 t I

t5'h-l

z . o

bole grains contain anomalously htgh Al contents, andthese analyses have been excluded from further discus-sion and calculation of average composition. Figure l0shows a general overlap in composition of analyses fromfine-grained enclaves and host granitoid. The only nota-ble difference between fine-grained enclaves and hostgranitoid is the greater Na content of hornblende in theenclaves. Amphibole from the mafic dike (CA84-65A)contains more total Al and alkalis and less Si than am-phibole in most fine-grained enclaves. The hornblendesamples from coarse-grained enclaves (BW84-27 andCA84-ll4) have significantly lower Fe/(Fe * Mg), andthey have lower contents of Al, Mn, Na, and K and greatercontents of Si and Ca. These data suggest equilibrationof amphibole in fine-grained enclaves and host and little(if any) equilibration of coarse-grained enclaves with theirsurroundings.

(-

Fig. 9. Biotite microprobe data. Numbers correspond to ab-brewiations of sample numbers given in Table 3 and rock typesin legend for Figure 10. Density ofdata rvithin envelopes is rep-resented by two samples, filled circles (CA84-45) and frlled squares(CA84-65A). Compositions of biotite in fine-grained enclavesfrom ilmenite-bearing granites (labeled ilmenite on right) aredistinct from those found in magnetite-bearing granodiorites(magnetite on left). In general, compositions of biotite are verysimilar for grains in fine-grained enclaves and corresponding hostgranitoids (see Table l); those in a mafic dike have a similarcomposition. Biotites from a coarse-grained dike have lower Fe/(Fe + Mg) and are not shown.

z . z

n e

o.2

0 .1

= 2.4

0,00.6

o.4LL

o.2

0.0

MAGNETITE ILMENITEenclave-\:-

20A

I granite-l_ro

65a l -

&t:,i -1To

Fel(Fe+Mg)

ALLEN: MAFIC ENCLAVES IN GRANITOIDS s83

f tete S-Continued

BW84-29fg enclave

CA84.45fg enclave

cA84-65Amafic dike

cA84-1 14cg enclave

Avgn : 4 7

srdstd Avgn : 1 5

Avgn : 6

std Avgn : 3

std

37.4315.753.32

17.710.60

12.340.030.269.080.263.880.21

100.46

5.5822.4180.350o.3722.2100.0762.7440.0050.0761.7280.1 230.446

0.750.900.300.430.060.410.040.180.460.050.05

1.22

0.077o.o770.0860.0320.0660.0080.0750.0060.0510.0840.0220.010

37.7415.233.11

17.310.63

12.080.100.26E.620.343.800.20

99.00

5.6872.3130.3900.3522.1810.0802.7130.0160.0761.6560.1600.446

1.020.23o.470.050.420.070.080.500.040.06

0.09s0.0270.0580.0060.0840.0110.022

0.3270.3822.3740.0522.5900.0120.040

0.670.230.15o.210.050.'120.040.04o.320.050.04

0.95

0.0390.0390.0480.0170.0490.0070.0420.0070.0110.0450.0250.004

36.8716.483.49

14.890.18

13.640.060.348.750.163.930.13

98.66

5.5022.498o.4200.3871.8740.0233.0410.0110.1 071.6650.0690.380

0.650.560.120.220.010.230.010.040.130.070.05

o.87

0.0640.0640.0720.0060.o240.0010.0390.0020.0040.0060.0410.002

Compositions in wtcA0.75 37.37

15.363.39

18.910.41

11 .580.080 .149 .150 .143.920.09

1.57 100.34Cations pol 24 O aioms0.091 5.6100.091 2.390

0.079 1.7510.020 0.0670.007 0.478

Plagioclase

The zonation patterns and anorthite contents of pla-gioclase from granitoids and mafic rocks are oscillatory

TleLE 6. Amphibole compositions

and variable in chemical trend (Fig. I l). Except for asmall volume enclave (approximately 25 cm3) in biotite-ilmenite granite (BW84-20A), mafic rocks have zonationpatterns that differ from their host granitoid. The zona-

BW84-23 BW84-25granodiorite granodiorito

BW84-29fg enclave

CA84-45tg enclave

cA84-654mafic dike

cA84-114 CA8+27€ enclave cg enclave

Avgn : 2 8

std Avgn: 4 ' l

std Avgn : 1 4

srd Avgn : 2 7

std Avgn : 1 0

std Avgn = 5

std Avgn : 5

std

sio,Al2ooTio,FeOMnOMgoCaONaroK.orH.o- o : F

Total

Sit4lAl

16lAl

TiFeMnMgCaNaKFFe/(Fe + Mg)N a + K

46.42 0.738.09 0.481.01 0.17

16.14 0.350.98 0.07

1 1.95 0.381 1.80 0.190.86 0.090.77 0.060.07 0.061.95 0.030.03

99.98 0.63

6.896 0.0681.104 0.0680.313 0.0610.113 0 .0182.006 0.0530.123 0.0092.646 0.0741.878 0.0340.249 0.0250.145 0.0110.065 0.0280.431 0.0110.395 0.028

45.42 0.948.60 0.681.21 0 .18

16.90 0.560.73 0.07'| 1.29 0.51

11.47 0.490.86 0.'t20.99 0.280.13 0.031.91 0.040.02

99.56 1.03

6.816 0.0931.184 0.0930.338 0.0830.137 0.0202.121 0.0790.093 0.0082.s26 0.0971.U4 0.0690.252 0.0350.189 0.0550.064 0.0260.457 0.0170.441 0.061

46.07 0.679.18 0.401.39 0 .11

16.78 0.360.54 0.03

11.61 0.3711.90 0 j21 .07 0 .150.97 0.050.09 0.051.98 0.030.06

101.52 0.52

6.765 0.0641.235 0.0640.353 0.0190.153 0.0132.061 0.0540.067 0.0042.s40 0.0681.872 0.0200.298 0.0460.182 0.0110.046 0.0290.448 0.0130.480 0.047

48.69 0.176.40 0.130.97 0.05

11.91 0.260.27 0.03

14.44 0.5312.22 0.070.99 0.330.58 0.010.10 0.091.99 0.050.08

98.52 0.79

7.172 0.0350.828 0.0350.283 0.0340.108 0.0061.467 0.0360.034 0.0043.170 0.0981.929 0.0070.233 0.0120.110 0.0030.048 0.0410.316 0.0110.343 0.014

47.32 1.358.44 1.431.O2 0.25

14.42 0.340.33 0.04

12.81 0.7512.11 0.240.66 0.090.73 0.120.04 0.032.O2 0.030.05

99.90 0.8i1

6.949 01471.051 0.147o.4't2 0.1140.113 0.028't.771 0.0460.041 0.0052.803 0.1451.906 0.0350.179 0.0140.138 0.0230.017 0.0140.388 0.0160.316 0.032

Compositions in wt9646.49 0.62 47.66 0.778.43 0.25 7.57 0.950.96 0.10 0.81 0,10

16.49 0.24 15.55 0.550.9s 0.04 0.99 0.06

1 1.97 0.28 12.26 0.4011.75 0.32 1 1 .57 0.361.02 0.11 0.98 0.100.84 0.05 0.69 0.080.1 1 0.11 0.24 0.081.95 0.05 1.91 0.050.01 0.15

101.06 0.82 100.39 1.25Cations per 24 O atoms

6.859 0.044 7.019 0.0681.141 0.044 0.981 0.0680.325 0.022 0.362 0.1030.106 0.011 0.090 0.0112.0u 0.031 1.916 0.0770.118 0.005 0.124 0.0072.632 0.054 2.691 0.0761.857 0.049 1.825 o.O|il0.295 0.029 0.279 0.0260.158 0.010 0.130 0.0150.069 0.048 0.113 0.0380.436 0.008 0.416 0.0130.453 0.030 0.408 0.028


TABLE 7. Representative plagioclase analyses

BW84-20A BW84-20A BW84_23Sample granite fg encl granodiorite

BW84-29 BW&L25fg encl granodiorite

cA84-114 CA84-654€ encl mafic dike

BW84-27cg encl

Analysis nosio,Al2o3FeOCaONaroK.O

Total

505762.6423.960.025.358 . 1 10.26

100.34

2.7621.2450.0010.2530.6930.0154.969

26.372.11 . 6

420563.4222.290.083.769.510.18

99.24

2.8241.1690.0030.1 790.8210.0105.006

17.781.31 . 0

129459.6325.910.097.246.200.23

99.30

2.6641.3640.0030.3470.5370.0134.928

38.759.91 . 4

138157.6627.880.109.s04.910.23

100.28

2.5651.4620.0040.4530.4240.0134.921

50.947.61 .5

Compositions in wt%1348 1492

60.20 60.s125.87 25.280.08 0.016.94 6.556.00 7.920.20 0.11

99.29 100.38CationsperSOatoms

2.681 2.682

1469 140059.61 57.4625.95 27.950.05 0.067.25 9.027.35 5.410.20 0.12

100.41 100.02

siAIFECaNaK

TotalAnAbOr

1.3580.0030.3310.518

1.3200.0000.3110.680

2.6471.3580.0020.3450.6fr10.0114.996

34.964.01 . 1

2.5621.4690.0020.4310.4680.0074.939

47.651.70.8

0.011 0.0064.902 4.999

38.5 31.260.2 68.21.6 0.6

tion pattern of plagioclase (phenocrysts and ground mass)in all enclaves is normal in contrast to a reversed trendfor plagioclase in granodiorites. The maintenance of thisdifferent zonation pattern in mafic rocks, and some dif-ferences in the range of anorthite contents (Fig. I 1) in-dicates that plagioclase in mafic rocks has not fully equil-ibrated with surrounding granitoids. A possible exceptionis plagioclase in the small enclave in granite.

Apatite

Apatite was analyzed in order to test for chemical dif-ferences between apatite with acicular and prismatic hab-its, and chemical differences between apatite from maficrocks (excluding coarse-grained enclaves) and apatite fromgranitoids. Representative and average analyses appearin Table 8. Differences of concentration occur for Ca, Mn,and F. A plot of MnO and F in wt9o (Fig. 12) shows that

Tmu 8, Apatite analyses, representative and average

acicular and prismatic apatite in biotite granite (BW84-20A) and biotite-hornblende granodiorite (BW84-23)contain more MnO and F than acicular apatite in a fine-grained enclave (BW84-29) and mafic dike (CA84-65A).Figure 12 also shows that acicular apatite from a smallenclave in granite (BW-20A) is intermediate in compo-sition and does overlap the compositions of acicular ap-atite from its host granite. A survey of apatite composi-tions from the literature indicates that apatite from maficrocks commonly contains less MnO and F than apatitefrom granitoids (see Wyborn, 1983; ke et al., 1973; Nash,1972). Thus, apatite compositions from mafic rocks ap-pear to record growth in a magma more mafic than thosein granitoids, and both acicular and prismatic apatite ingranite are chemically indigenous to granite. It is possiblethat apatite from the small enclave in granite (BW84-20A) partially equilibrated with its surroundings.

Sample

Analysis no.

Prismatic in graniteBW84-20A

Prismatic in granodioriteBW84-23

Acicular in graniteBW84-20A

7042 Avgn : 2 5

Avgn : 3

7179 Avgn : 1 4

sio,PrOuFeOMnOCaOF- o : F

Total

o.2240.710.'170.50

55.132.901.22

98.41

0.022.850.010.044.880.767.79

o.2439.82o.240.46

54.423.481.47

97.19

0.022.820.020.034.870.927.76

0.10o.770.180.070.81o.44

0.4951.433.271.38

97.75

0.020.480.010.010.420.03

0.97

0.000.010.000.000.020.01

0.4040.470.100.58

53.053.481.47

96.61

0.2740.920.330.41

53.353.071.29

97.04

0.022.890.020.034.770.817.73

0.080.980.160.110.990.46

2.79

0.010.040.010.010.09o.12

Composation in wt%0.26 0.23

43.59 43.650.0s 0.06

SiPFeMnCaF

Total

0.4751.733.231.36

0.99 97.97

0.010.030.010.010.08o.12

Cations per 13 O atomg0.02 0.022.99 3.000.00 0.000.03 0.034.50 4.47

0.032.860.010.044.740.927.68

0.837.55

0.847.53

granitoids=23, 25f.g. enclaves=29, 45c.g. enclaves=27, 11mafic dike= 65A


0.50 n

585

60

50Et .so

ltJ 1

IFuo -Z J

bR

0.40Fel(Fe+Mg)

Fig. 10. Amphibole microprobe data. Numbers and filledsquares as in Figure 9. A1l analyses are ofcalcic magnesio-horn-blende except some from CA84-65A (mafic dike) which areedenite. The label f.g. : fine-grained; c.g. : coarse-grained.

Ap.lrrrB Rb-Sr rsoroPrc DATA

The presence of some chemical contrasts in apatite frommafic and felsic rocks suggests isotopic contrasts mightalso exist. Apatite was chosen for analysis because itstextures suggested early crystallizatiot, perhaps prior topartial homogenization through magma mixing, assimi-lation, diffusion, etc. The two apatite habits from graniteare in isotopic equilibrium with each other and the wholerock (Table 9; SRI : 0.7084 + l). Acicular apatite froman enclave (BW84-29), however, is more radiogenic thanthe bulk enclave (SRI : 0.7074 + I and 0.7069 t I,respectively). The surrounding granodiorite @W84-23) isin isotopic equilibrium with the bulk enclave (SRI :0.7071 r l ) .

An apparent age of 67 Ma can be calculated from datafrom apatite and the whole enclave. This age is not sig-nificant in the history of the Turtle pluton (Allen, 1989).This result suggests that apatite gained radiogenic Sr or

T aEte g-Continued

Fig. 11. Plagioclase compositions. Range is given by verticalline, I standard deviation by cross bars, and mean by dot. Num-ber ofanalyses is given below the line. Host granitoid-mafic rockpairs are given together.

lost Rb relative to the whole enclave. Diffusion acrossthese narrow grains in a geologically reasonable time canaccount for the more radiogenic apatite (Watson et al.,l 985).

DrscussroNThe chemistry and mineral content of mafic rocks

within the Turtle pluton suggest that three mafic rocktypes have equilibrated with their local environment todifferent degrees. Fine-grained enclaves have been affect-

20

10

@

o : -fo oo _ f

a o=o oP

€ = € €e m c o c o m @f f + + + +l t t t l l

N ) N N ) N ) o )( , J @ ( r ! : u l

+

o : -l

oo

N N

5 4 6 8u l @= =o @F +t l

I$ N)

a .bf '

! oO oI -= l' l

I- -+-x l

e47

N=NORMAL ZONATION

R=REVERSED ZONATION

orO

of

(!

N-t-?t6

l

oo-o

ToI-r

5

Acicular in enclaveBW8420A

Acicular in enclaveBW84-29

Acicular in mafic dikecA84-654

7148 Avgn : 1 4

std 7115 Avgn = ' 1 4

Avgn : 5

std

0.4042.670.490.45

52.192.42't.02

97.60

0.032.970.030.034.600.637.66

0.4642.110.290.29

53.062.741 . 1 6

97.78

0.042.920.020.024.680.727.68

o.213.51o.240.121.630.56

1.59

0.020.160.020.010.300.17

0.2343.670.060.04

54.782.060.87

99.97

0.022.980.000.004.730.537.74

0.370.800.170.020.510.24

2.12

0.3042.200.030.06

54.812.010.85

98.56

0.032.940.000.004.830.s27.80

0.4741.480.060.08

54.011.790.76

97.13

0.042.930.000.014.8ril0.477.81

0.26o.720.080.030.500.16

1.74

0.020.030.010.000.030.04

Composilion in wt960.66

42.820.170.09

54.411.860.79

99.23Caiions per 13 O atom3

0.052.950.010.014.740.487.77

0.030.040.010.000.040.06


ed the most, and coarse-grained enclaves and mafic dikes,less so. The degree of equilibration through mixing, min-gling, or diffusion and reaction depends on several fac-tors: proportion of mafic to felsic magma (Sparks andMarshall, 1986), transport properties (Lesher, 1990), en-claves size, average grain size, and residence time in thepluton (Frost and Mahood, 1987).

Coarse-grained enclaves are relatively large bodies thatare chemically and isotopically distinct from host grano-diorites and other mafic rocks, hence they are not cu-mulates or restites related to the Turtle pluton. However,the Al contents of amphibole rims from one enclave sug-gest equilibration at pressures similar to that of the pluton(Table l0). Most probably these coarse-grained enclavesrepresent accidental inclusions of a mafic rock type in thegranitoid magma.

Some isotopic and mineral chemical differences remainbetween mafic dikes and surrounding granodiorites, per-haps resulting from the relatively late introduction ofthedikes into the plutonic environment, compared to fine-grained enclaves. The greater alkali content ofbiotite andamphibole (edenite) in dikes, as compared to fine-grainedenclaves, may reflect a distinct source. A lower SRI anda higher e"u(t) in the dikes as compared with the granit-oids suggest less crustal involvement in the dikes.

Fine-grained enclaves have thoroughly interacted withthe surrounding granitoids. This is reflected in whole rockcompositions, mineral abundances, and mineral chem-istry. Different minerals have equilibrated with their sur-roundings to different degrees; biotite has most fullyequilibrated, amphibole and apatite less so, and plagio-clase maintains distinct compositions and zoning pat-terns.

The modal abundance of biotite in fine-grained en-claves (10-200/o), igneous textures of biotite, and the me-dium- to hlgh-Kro composition of these enclaves suggestthat components (particularly KrO) from host granitoidswere added to the enclaves at temperatures above thesolidus. Overlap of compositions of biotite from enclavesand host rocks, and similar Rb contents of whole rockenclave and host granitoid, suggest continued equilibra-tion of biotite (the Rb-controlling phase in enclaves).Equilibration of biotite is to be expected given its re-ported behavior in igneous and metamorphic rocks underhydrous conditions (Brimhall et al., 1983; Moore andCzamanske, 1973).

Tlale 9. Apatite and whole rock Rb-Sr isotopic data

67Sr/sSr Rb (ppm) Sr (ppm) 67Rb/ssr 'Sr/ssr;

Amphibole compositions from enclave and host par-tially overlap, and the total Al contents of the two rocktypes are similar. For selected analyses near rims, and inbiotite-free amphibole grains, pressures calculated fromAl contents of most fine-grained enclaves and granitoidsare indistinguishable (Table l0) and, consequently, sug-gest equilibration at similar pressures. Note that the ap-propriate assemblage for use of hornblende geothermom-etry is available, except where the absence ofpotassiumfeldspar is noted.

The low anorthite content of plagioclase (An : 28-38)from a fine-grained enclave of basaltic composition(BW84-29) indicates lack of equilibration and a possiblexenocrystic origin.

No bulk mixing model of dike and granitoid can ac-count for the range ofobserved fine-grained enclave com-positions in the Turtle pluton. In particular, no bulk modelof mafic dike plus mineral components from granitoidscan account (l) for SRI of some enclaves exceeding thatof their hosts or (2) for the dispersion of enclave isotopiccompositions away from the mixing line defined by gran-itoids on isotopic plots (Fig. 8).

Assimilation of granitoids by dikes followed by frac-tional crystallization can account for the range of enclavegeochemical data (including Rb-Sr isotopic ratios), butthe assimilant cannot be the local host rock in all casesbecause some enclaves are more radiogenic than theirhosts. Addition of radiogenic Sr through weathering oralteration of biotite to produce SRI higher in enclavesthan in host granitoids is unlikely given the similar modalabundances ofbiotite in both rock types. The higher SRIof the enclaves probably indicates interaction with a moreradiogenic source than their present hosts in the Turtlepluton.

The dispersion of enclave data from the mixing line onisotope plots defined by the granitoids (Fig. 8) could rep-resent assimilation and fractionation or a multiplicity ofsources, or it could be the result of partial equilibrationof enclaves with granitoids through diffirsion. Results froma recent experimental study of elemental and isotopic dif-fusion of Sr and Nd near an interface of felsic and maficmagma suggest complicated diftrsion paths, more rapidequilibration of isotopic ratios than elemental concentra-tions, and equilibration rates of875r/865r twice as fast ast43Nd//r44Nd (Lesher, 1990). Thus in natural settings, en-claves modified by interaction with granitoid magmas

Trau 10. Hornblende geobarometry

Calculated P (kbar)Samole No. ofnumber analyses

Avg. StandardAl., deviation

Whole rockac. apatitepr. apatite

Whole rockac. apatite

Granite (CA85-5)0.70985 97.1 338.1 0.83 0.70830.70881 17.5 271.7 0.19 0.70850.70858 2.3 112.7 0.06 0.7085

Fine-grained enclave (8W84-29)0.70833 90.5 346.4 0.76 0.70690.70778 5.0 81.7 0.18 0.7074

BW84-23BW84-29BW84-25BWU-27cA84-45cA84-654-

1 .51 0 .131.45 0.041.50 0.131 .34 0.101.27 0.091.56 0.O7

3.7 3.8 2.93.4 3.4 2.73.6 3.7 2.92.8 2.8 2.22.5 2.4 1.93.9 4.0 3.1

201 01 7496

A/ote: Abbreviations: ac. : acicular, pr. : prismatic.* Assumes a crystallization age of 130 Ma (Allen, 1989).

,Vote.' A : Hammarstrom dnd Zen (1986), +3 kbar; B : Hollister et al.(1987), +1 kbar; g: Johnson and Rutherford (1989), +0.5 kbar.

- Lacks potassium feldspar.

ALLEN: MAFIC ENCLAVES IN GRANITOIDS ) 6 /

0.7

0.6

A R

xE o.+

5 0.3v .z

0 .1

O equant, graniteA acicular, granite- equant, granodioriteO acicular, enclave in gr OA acicular, enclave inI acicular, mafic dike

A . i-

I

o

o'oo'1 2 3 4

F wt%

Fig. 12. Apatite compositions as represented by wto/o MnOand F. Apatite of both morphologies in granite (BW84-20A) andgranodiorite have similar compositions, and the small enclavein the granite has a similar F range but contains less MnO. Acic-ular apatite from another fine-grained enclave (8W84-29) andmafic dike (CA84-65A) have similar MnO and F concentrationsbut distinctly lower than those of other samples.

through diffusion may not have the compositions of sim-ple mixtures of granitoid and basalt or even lie alongassimilation-fractionation paths. If so, determination ofa enclave source based on isotopic ratios and, perhaps,concentrations of other elements would be very difficult;nonetheless, these experiments do suggest that Nd iso-topic data are more likely to retain source informationthan Sr isotopic data.

Although overlapping mineral compositions, KrO andRb contents, and SRI have been interpreted as signs oflocal interaction offine-grained enclaves and Turtle plu-ton granitoids, these data could also be interpreted toindicate a genetic relationship ofthe enclaves to the en-closing pluton. However, that enclaves have higher e"o (l)than their host granitoids indicates distinct and moreprimitive sources for the enclaves.

Rb-Sr isotopic and geochemical relationships similarto those described in this study have been reported forfine-grained enclaves and ganitoids of the Criffell andStrontian plutons of Scotland (Halliday et al., 1980;Holden et al., 1987). Relative to host granodiorites, fine-grained enclaves have greater or equal SRI (within 0.0004,except one pair), greater or equal concentrations of Rb,and lesser or equal concentrations of Sr. Though no Srisotopic signature of mantle derivation remains in theseenclaves, Nd-Sm isotopic data suggest a mantle sourcefor enclaves [e"u(l) of enclaves is +0.4 > hosts]. Theyattributed maintenance of Nd isotopic composition inenclaves to behavior of this relatively immobile elementin apatite.

CoNcr,usroNs

Determination of sources of mafic enclaves has beenhampered by the interaction ofenclaves and surroundinggranitoids. Evidence oflocal interaction presented in thisstudy includes similar mineral assemblages, similar min-eral compositions of amphibole and biotite, whole rock

Rb contents, and similar SRI of enclaves and granitoids.Equilibration of modally significant biotite and horn-blende probably affects whole rock major and trace ele-ment compositions and Sr isotopic ratios. Partial equili-bration of an unrelated mafic magma in a granitoid plutoncan account for the similar geochemical characteristics ofenclave and host that have been attributed to mixing ofdistinct batches of mafic magmas with a felsic magma togenerate a range of intermediate compositions (Cocirta etal., 1989), and to incomplete removal of restite (Chen etal., 1989). Petrogenetic indicators that equilibrate rela-tively rapidly at high temperatures, such as many majorand trace elements and Sr isotopes, cannot be used toestablish enclave sources. Instead petrogenetic indicatorsthat equilibrate more slowly, such as Nd isotopes, aremore reliable.

AcxNowr,nocMENTS

This manuscript was greatly improved through early discussions withCalvin Barnes and John Reid, and through revielvs by C.E. ksher andTom Frost. The author thankfully acknowledges use of facilities at VPIand SU during her tenure as a graduate student, particularly the use ofthe isotope lab. Discussions with A.IC Sinha were key to eompletion ofthis study. Joe Wooden graciously granted access to his lab at the U.S.Geological Survey in Menlo Park, California.

RnrnnrNcns crrEDAllen, C M. (1989) Petrogenesis of the reversely zoned Turtle pluton,

southeastern California, 374 p. Ph.D. dissertation, Viryinia PolytechnicInstitute and State University, Blacksburg, Virginia.

Barbarin, B. (1988) Field evidence for successive mixing and minglingbetween the Piolard Diorite and the Saint-Julien-la-Vetre Monzogran-ite (Nord-Forez, Massif Central, France). Canadian Journal of EarthSciences, 24,49-59.

Barnes, C.G., Allen, C.M., and Saleeby, J.B. (1986) Open- and closed-system characteristics of a tilted plutonic system, Klamath Mountains,California. Journal of Geophysical Research, 9 l, 607 3-6090.

Bateman, P.C., and Chappell, B.W. (1979) Crystallization, fractionation,and solidification of the Tuolumne Intrusive Series, Yosemite NationalPark, California. Geological Society of America Bulletin, 90, 465-482.

Bateman, P.C., Clark, L.D., Huber, N.K., Moore, J.G., and Rirehart,C.D. (1963) The Sierra Nevada batholith-A synthesis ofrecent workacross the central part. U.S. Geological Suwey Professional Paper 414D,D1-D46.

Brimhall, G.H., Agee, C., and Stoffregen, R. (1985) The hydrothermalconversion ofhornblende to biotite. Canadian Mineralogist, 23,369-379.

Canta$el, J.-M., Didier, J., and Gourgaud, A. (1984) Magma mixing:Origin of intermediate rocks and "enclaves" from volcanism to plu-

tonism. Physics of the Earth and Planetary Interiors, 35, 63-76.Chen, Y.D., Price, R.C., and White, A.J.R. (1989) Inclusions in three

S-type $anitos from southeastem Australia. Joumal ofPetrology, 30,l l 8 l - 1 2 1 8 .

Cocirta, C., Orsini, J.B., and Coulon, C. (1989) Exemples de melange demagmas en contexte plutonique: ks enclaves des tonalites-grano-diorites du massifde Bono (Sardaigne septentriona). Canadian Journalof Earth Sciences, 26, 1264-129l.

Didier, J. (1973) Granites and their enclaves, 393 p. Elsevier, Amsterdam.-(1987) Contribution of enclave studies to the understanding of

origin and evolution ofgranitic magmas. Geologische Rundschau, 76,4 l -50.

Eberz, G.W., and Nicholls, I.A. (1990) Chemical modification of enclavemagma by post-emplacement crystal fractionation, diftision and meta-somatism. Contributions to Mineralogy and Petrology, 104,47-55.

Frost, T.P., and Mahood, G.A. (1987) Field, chemical, and physical con-straints on mafic-felsic magma interaction in the Lamark Granodiorite,


Sierra Nevada, California. Geological Society ofAmerica Bulletin. 99.272-29t .

Gill, J.G. (1981) Orogenic andesites and plate tectonics, 390 p. Springer-Verlag, Berlin.

Halliday, A.N., Stephens, W.E., and Harmon, R.S. (1980) Rb-Sr and Oisotopic relationships in 3 zoned Caledonian granitic plutons, SouthernUplands, Scotland: Evidence for varied sources and hybridisation ofmagmas. Journal of the Geological Society of London, 137 , 329-34g.

Hammarstrom, J.M., and Zen, E-an (1986) Aluminum in hornblende: Anempirical igneous geobarometer. American Mineralogist, 7 l, 1297-1313.

Hawthome, F.C. (1981) Crystal chemistry of the amphiboles. In Miner-alogical Society of America Rewiews in Mineralogy, 9A, 1-102.

Holden, P., Halliday, A.N., and Stephens, W.E. (1987) Neodymium andstrontium isotope content of microdiorite enclaves points to mantleinput to ganitoid production. Nature, 330, 53-56.

Hollister, L.S., Grissom, G.C., Peters, E.K., Stowell, H.H., and Sisson,V.B. (1987) Confirmation of the empirical correlation of Al in horn-blende with pressure of solidification of calc-alkaline plutons. Ameri-can Mineralogist, 72, 231-239.

Johnson, M.C., and Rutherford, M.J. (1989) Experimental calibration ofthe aluminum-in-hornblende geobarometer with application to LongValley caldera (California) volcanic rocks. Geology, l7 , 837-841.

Jurinski, J.B., and Sinha, A.K. (1989) Igneous complexes within the CoastalMaine Magmatic Province: Evidence for a Silurian tensional environ-ment. Geological Society of America Abstracts with programs, 21, 25.

Kumar, S.C. (1988) Microgranular enclaves in granitoids: Agents of mag-ma mixing. Journal of Southeast Asian Earth Sciences, 2. l19-l}l.

ke, D.E., Van Loenen, R.E., and Mays, R.E. (1973) Accessory apatitefrom hybrid granitoid rocks of the southern Snake Range, Nevada.Journal ofResearch ofthe U.S. Geological Survey, 1, g9-9g.

ksher, C.E. (1990) Decoupling of chemical and isotopic exchange duringmagma mixing. Nature, 344, 2j5-237.

Lofgren, G. (1974) An experimental study of plagioclase crystal mor-phology: Isothermal crystallization. American Journal of Science. 274.243-27 4.

Moore, W.J., and Czamanske, G.K. (1973) Compositions of biotite fromunaltered and altered rnonzonite rocks in the Bingham Mining District,Utah. Economic Geology, 68, 269-27 4.

Nash, W.P. (1972) Apatite chemistry and phosphorous fugacity in a dif-ferentiated igneous intrusion. American Mineralogist, 57, 87 7 -886.

Norrish, K., and Chappell, B.W. (1977) X-ray fluorescence spectroscopy.In J. Zussman, Ed., Physical methods in determinative mineralogy, p.201-272. Wiley, New York

Reid, J.B., Evans, O.C., and Fates, D.G. (1983) Magma mixing in graniticrocks of the central Sierra Nevada, Califomia. Earth and PlanetarySciences. 66. 243-261.

Solberg, T.N., and Speer, JA. (1982) QALL, a l6-element anallticalscheme for efrcient petrologic work on an automated ARL-SEMQ;application to mica reference samples. Microbeam Analysis 1982,422-426.

Sparks, R.S.J., and Marshall, L.A. (1986) Thermal and mechanical con-straints on mixing between mafic and silicic magmas. Journal of Vol-canology and Geothermal Research, 29,99-124

Stewart, D.B., Arth, J.G., and Flohr, M.J.K. (1988) Petrogenesis of theSouth Penobscot Intrusive Suite, Maine. American Journal of Science,288-4, 7 5-tt4.

Vernon, R.H. (1983) Restite, xenoliths and microgranitoid enclaves ingranites. Journal and Proceedings of the Royal Society of New SouthWales, l l6 .77-103.

Watson, E.B., Harrison, M.T., and Ryerson, F.J. (1985) Diffusion of Sm,Sr, and Pb in fluorapatite. Geochimica et Cosmochimica Acta, 49,18 I 3-1 823.

Waugh, B.J. (1985) The origin of mafic enclaves rvithin a granodioriticpluton of the Turtle Mountains, CA, 50 p. B.S. thesis, Carleton College,Northfreld, Minnesota.

White, A.J.R., and Chappell, B.W. (1977) Ultrametamorphism and gran-itoid genesis. Tectonophysics, 43, 7 -22.

Wyborn, D. (l 983) Fractionation processes in the Boggy Plain zoned plu-ton, 301 p. Ph.D. dissertation, Australia National University, Canber-Ia.

Wyllie, P.J., Cox, K.G., and Biggar, c.M. (1962) The habit of apatite insynthetic systems and igneous rocks. Journal of Petrology, 3, 238-243.

Mer.n;scnrpr REcETVED FrsR.uARy 13, 1990MeNuscmyr ACcEFTED JINUARy 16, l99l

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Local equilibrium of mafic enclaves and granitoids of the ... · American Mineralogist, Volume 76, pages 574-588, 1991 Local equilibrium of mafic enclaves and granitoids of the Turtle